refactor: Major SDK architecture refactoring and API consolidation

- Consolidate separate buy/sell modules into unified trading interface
- Remove protocol-specific buy/sell files (bonk, pumpfun, pumpswap)
- Add new trading constants and utility functions
- Simplify API with unified buy/sell methods supporting multiple protocols
- Enhance documentation with comprehensive examples and usage guides
- Add balance checking and token account management utilities
- Improve code organization and maintainability

src/constants/trade.rs

@@ -0,0 +1,9 @@
+pub mod trade {
+    pub const DEFAULT_SLIPPAGE: u64 = 1000; // 10%
+    pub const DEFAULT_COMPUTE_UNIT_LIMIT: u32 = 78000;
+    pub const DEFAULT_COMPUTE_UNIT_PRICE: u64 = 500000;
+    pub const DEFAULT_BUY_TIP_FEE: f64 = 0.0006;
+    pub const DEFAULT_SELL_TIP_FEE: f64 = 0.0001;
+    pub const DEFAULT_RPC_UNIT_LIMIT: u32 = 1000000;
+    pub const DEFAULT_RPC_UNIT_PRICE: u64 = 500000;
+}